1. Field of the Invention
This invention relates to an automatic color printing system, and more particularly to a color printer and a method of automatically determining exposure in a color printer. More specifically, the present invention pertains to a color printing method in which a predetermined color is first detected in a color original (negative or positive color film) and when the color has been detected, the exposure is determined to print the color in a predetermined desired color balance and density.
2. Description of the Prior Art
Throughout the specification, the term "color printer" should be interpreted not only as a color printer in which a measuring portion to measure the density and color balance of a color original to obtain necessary information and a printing portion to print a color image on a color paper are connected on-line with each other, but also as a color printing system comprising a measuring portion which has a recording means such as a punched tape or a magnetic tape and a printing portion separated from the measuring portion to print color images according to the recorded information.
In determining exposure in color printers, various methods have been known and practically employed. A well-known printing system in which the printing light source intensity is adjusted during red, green and blue exposures to levels which will normalize the resulting integrated transmittances to a near-neutral color balance, i.e., "gray," is based on U.S. Pat. No. 2,571,697, Evans. This printing system produces satisfactory results from a large majority of the negatives of a given type of film. It has also been known in the art to adjust the rate of correction for red, green and blue exposures based on a linear combination of the red, green and blue large area transmission densities (LATD) of the original to be printed.
When the LATD system is used, the exposure time Ti for each color is represented by the following equation EQU log Ti=.alpha.iDi+.beta.i
where .alpha. and .beta. are numerical constants and Di is the LATD of each color (i indicated red, green or blue).
Since the above described conventional printing systems are based on the integrated transmission measurements conducted over the whole area of the original, the obtained prints are not always satisfactory. For instance, if the background of the principal subject matter is primarily red (red curtain or furniture), green (green grass or foliage) or blue (blue sky or water), color correction based only on the aforesaid LATD system is unsatisfactory. This problem is known as "color failure."
Further, if the background of the principal subject matter is of particularly high or low brightness, the conventional correction based on the integrated transmission density does not give satisfactory results. For example, when the principal subject matter has been photographed with a back light or in a spotlight conventional correction will give unsatisfactory results. This is known as "density failure."
According to the inventors' tests, in the color printing process using the LATD printing system the yield of satisfactory prints is about 70% of all the prints obtained.
It has also been known in the prior art to determine the exposure in a color printer based on the measured densities of divided areas of color originals in which the entire area of the original is divided into upper and lower, right and left, and central and peripheral sections. The exposure is determined based on a combination of the LATD and the densities of the divided areas. In this system, the yield of satisfactory prints is somewhat raised. However, since the density of the principal subject matter is not accurately measured in this system, the correction is not always effected in the most desirable manner.
According to the inventors' investigation, about 80% of the color originals have figures as the principal subject matter. Further, in looking at printed color photographs, it is well known that most people are concerned about the faces of the figures. Therefore, in printers, it is desirable that the faces of the figures be printed in a good condition. An exposure controlled to obtain a good flesh color in the areas of flesh color might be improper for areas other than the flesh color areas. However, even so, the yield of satisfactory prints is higher, if the exposure control is based on the flesh color areas.
Further, in most color originals is photographed trees, blue skies, blue sea, or in winter snows, together or without the human figure. Since these particular subjects are well known and the color thereof is well memorized, the photographers are also interested in the reproduction of the color of these particular subjects. These colors are, therefore, also desired to be reproduced in the desirable color which may not be automatically obtained by the conventional LATD method.
Thus, it is proposed to print color originals based on the color of a particular subject when the originals contain over a certain number of points of the color of the particular subject. If the originals have no areas of the particular color, the exposure may be controlled according to the conventional LATD printing system or the like.